The advantages of reduced cost and bandwidth savings of carrying FoIP networks are associated with some QoS issues that are unique to packet networks and can affect the reliability of the fax transmission.

Timing
A major issue in the implementation of FoIP networks is the problem of inaccurate timing of messages caused by delay through the network. The delay of fax packets through a packet network causes the precise timing that is required for many portions of the fax protocol to be skewed and can result in the loss of the call. The FoIP protocol in the IWF must compensate for the loss of a fixed timing of messages over the packet network so that the T.30 protocol operates without error.

There are two sources of delay in an end-to-end, FoIP call: network delay and processing delay.

1. network delay—This is caused by the physical medium and protocols that are used to transmit the fax data and by buffers used to remove packet jitter on the receiving end. This delay is a function of the capacity of the links in the network and the processing that occurs as the packets transit the network. The jitter buffers add delay when they remove the packet-delay variation of each packet as it transits the packet network. This delay can be a significant part of the overall delay, as packet-delay variations can be as high as 70 to 100 milliseconds in some frame-relay networks and even higher in IP networks.

2. processing delay—This is caused by the process of demodulating and collecting the digital fax information into a packet for transmission over the packet network. The encoding delay is a function of both the processor execution time and the amount of data collected before sending a packet to the network. Low-speed data, for instance, is usually sent out with a single byte per packet, as the time to collect a byte of information at 300 bps is 30 milliseconds.

Jitter
Delay issues are compounded by the need to remove jitter, a variable interpacket timing caused by the network that a packet traverses. An approach to removing the jitter is to collect packets and hold them long enough so that the slowest packets to arrive are still in time to be played in the correct sequence. This approach, however, causes additional delay. In most FoIP protocols, a time stamp is incorporated in the packet to ensure that the information is played out at the proper instant.

Lost-Packet Compensation
Lost packets can be an even more severe problem, depending on the type of packet network that is being used. In a VoIP application, the loss of packets can be addressed by replaying last packets and other methods of interpolation. A FoIP application, however, has more severe constraints on the loss of data, as the fax protocol can fail if information is lost. This problem varies, depending on the type of fax machine used and whether error-correction mode is enabled.

Two schemes that are used by FoIP software to address the problems of lost frames are as follows:

• repeating information in subsequent frames so that the error can be corrected by the receiver's playout mechanism
• using an error-correcting protocol such as TCP to transport the fax data at the expense of added delay